1. This study was designed to test the hypothesis that simultaneous non-invasive assessment of the circadian variations in both intermittent arterial pressure and the continuous 24 h changes of spectral markers of cardiac neural control could provide new information on cardiovascular regulatory mechanisms, in hypertensive patients and normotensive subjects. To test this hypothesis we studied 18 subjects with mild hypertension and 11 normotensive subjects in whom we recorded simultaneously non-invasive intermittent arterial pressure and Holter electrocardiogram for 24 h. We also studied the same subjects during resting and standing conditions in the clinical laboratory. 2. The normalized power of the low-frequency (approximately 0.1 Hz) spectral component of R-R interval variability, considered mainly a marker of sympathetic drive to the sino-atrial node, was, at rest, significantly higher in the hypertensive than in the normotensive subjects, as already reported. Moreover, the values of the low-frequency component at rest recorded in the clinical laboratory were significantly correlated with those obtained from ambulatory recording during night rest. The decrease in the values of arterial pressure during the night-time was accompanied by a reduction in the power of the low-frequency component only in the case of normotensive subjects. Accordingly, the slope of the regression of the low-frequency component as a function of systolic arterial pressure during ambulatory recordings was steep in normotensive subjects and flat in hypertensive subjects.(ABSTRACT TRUNCATED AT 250 WORDS)